You can do that really quickly from the Plesk GUI too. Even easier than throwing up an .htaccess file.
From your admin console go to the domain in question, then click on the Directories button. Plug in the "/webstat" directory, then set up a username/password for it.
Personally, I done this as I as part of the domain setup process all of my sites since the webalizer stats are freely available to anybody who points their browser to
http://www.domain.com/webstat if you don't. Normal surfers just don't need access to that IMHO.
Squire